M3 fits snugly through the fan holes on a PA120.X but since the holes aren't threaded you'll need to use a nut. On the other hand, you can use self-tapping screws.
2 - Provided Screws
You get 1x spare bleed valve screw, and 4x small black self tappers to attach fan to rad. If you purchase the radiator as part of a complete kit, you also get 4 large self tappers for attaching fan to case.
Please be aware that if you use the black self tappers to mount the radiator directly to your case, they will likely puncture the core. This is not covered by warranty. If you wish to mount the radiator directly to your case, you should source your own screws suitable for the task. Ensure whatever screws you use aren't of a length where the screw's tip will penetrate the core's tubes once installed.
The ThermoChill black selftappers are sized to go thru fan & neoprene gasket, then into radiator, not for case directly to radiator.
Small black self-tappers used are 6x1/2" Sheet Metal Screws.
For closed corner fans such as the Nexus and YateLoon, use 3.5x38mm Sheet Metal Screws.